1. Field of the Technology
The present invention relates to signal transmission technology, and more particularly, to a method and system for reducing delay difference of differential transmission.
2. Background of the Invention
Along with the rapid development of high speed interconnection technology, the speed of channel transmission has become increasingly higher and has reached 10 Gbps. Generally, a pair of differential signals is adopted for high speed signal transmission. The pair of differential signals includes two signals with opposite polarities, one of which is defined as positive signal, i.e., P signal while the other is defined as negative signal, i.e., N signal. Therefore, the differential signals should be transmitted on two channels, that is to say, there are two channels for such signal transmission. At the receiving end, the received signal is obtained through subtracting the positive signal from the negative signal and then subsequent processes such as level decision are applied to the obtained signal.
Ideally, the two differential signals with opposite polarities will be transmitted to a certain point of the channel between the transmitting end and the receiving end simultaneously. Therefore, the two differential signals will be transmitted to the detector of the chip at the receiving end simultaneously. Accordingly, in order to ensure the reliability of signal transmission, the two signals with opposite polarities should have the same delay during differential signal transmission, as shown in FIG. 1, which means that the delay difference of differential transmission should be zero, and only in this case, the optimum received signal can be obtained through subtracting the positive signal from the negative signal, as shown in FIG. 2.
However, due to certain reasons such as anisotropy of the printed circuit board materials and discrepancy in real distance of differential cable, the transmission delay of the two signals with opposite polarities are inconsistent during the differential signal transmission, which means that the delay difference of the differential transmission is not zero.
FIG. 3 is a schematic diagram of differential signals with non-zero delay difference of differential transmission at the receiving end. FIG. 4 shows a schematic diagram of the received signal obtained through processing the two differential signals of FIG. 3. Obviously, the signal obtained through differential transmission shown in FIG. 4 is not an expected signal.
Therefore, the bigger the delay difference of differential transmission is, the heavier the signal jitters will occur at the receiving end, and the larger the error of the optimum sampling points for Clock and Data Recovery (CDR) will become. Deterioration of the optimum sampling points may further increase the Bit Error Ratio (BER) of received signal at the receiving end, and deteriorate the system performance. As to the signal with relatively high speed, any deterioration may result in a sharp increase of BER, even to the extent of abnormal operation of system.
In order to solve the problem, a method for reducing delay difference of differential transmission is put forward at present. The main idea of the method is implementing delay compensation to P signal and N signal at the receiving end. As shown in FIG. 5, the delay difference compensation devices include two delay controllers, two delay modules, a subtracter, an error generating circuit and a threshold level decision (SLICER). The adaptive compensation for delay difference of differential transmission is to calculate the difference between the signals before and after the SLICER and obtain an error component. The device transmits the error component to the two delay controllers at the P end and the N end, and the delay controllers further determine delay control components based on the error component. The delay modules determine delay adjustment components at the P end and the N end according to the delay control components, and further compensate the delay difference of differential transmission generated in the channels.